Used oils, particularly those used as lubricating oils, are normally considered waste once they have been used for their intended purpose, (e.g., crankcase oils etc.) as lubricants and are normally either discarded or burned, either disposal method having deleterious environmental ramifications associated therewith.
Consequently, as an alternative, methods for reclaiming the used oil molecules have been developed, both to reduce the environmental hazard associated with either disposal or burning of the oil, and to conserve valuable molecules which can be refined into basestocks having a quality equivalent to virgin basestocks. Acid/clay treating is of diminishing applicability due to the complexity of the additives employed in modern lubricating oils, as well as in light of the sludge disposal problem associated with acid/clay treatment.
The Phillips (PROP) process combines chemical demetallization with clay/hydrotreating as finishing steps. Used oil is mixed with an aqueous solution of, for example, diammonium phosphate which reacts with metal contaminants to form metallic phosphates which separate from both water and oil due to low solubility. The demetallized oil, after filtration using filter aid and heating, is contacted with a guard bed of clay and hydrotreated over a catalyst, for example Ni/Mo, and stripped (See U.S. Pat. No. 4,151,072).
The chief disadvantage of the PROP technology is the disposal of a large amount of solid wastes associated with the filter cake and spent clay. In addition, a large amount of waste water, originated from the chemical solution, is generated by this technology.
Vacuum distillation followed by hydrofining has been proposed in the literature for waste oil re-refining. The process is characterized by pollution-free operation without incurring sludge and oily-clay wastes. However, the drawback of this process is that contaminants in the distillate, originated from lube additives and/or degraded oil, cannot be removed to a low enough level during the distillation step. As a result, hydrofiner fouling becomes a serious problem.
Dewatering-defueling/solvent extraction/distillation/clay contacting and/or hydrofinishing has also been suggested (See U.S. Pat. Nos. 3,919,076, 4,073,719, 4,073,720). In these processes, various kinds of extraction solvent e.g., propane or a mixture of alcohol and ketone, are used to reduce coking and fouling precursors in the dewatered/defueled waste oil prior to the vacuum distillation. The resulting distillate is further upgraded by clay contacting and/or hydrofinishing. The chief disadvantage of this approach is the complexity of the solvent recovery systems which require high energy consumption, and generate waste chemicals via leakage from the unit.
In addition to the above-mentioned waste oil re-refining technologies, distillation of waste oil in a thin-film or wiped-film evaporator (TFE or WFE) to recover lube distillate is also known in the art ("Recent Technology Development In Evaporative Re-Refining of Waste Oil", Bishop and Arlidge; "Thin-Film Distillation as a Tool in Re-Refining Used Oil", Pauley, both of these articles in Third International Conference on Waste Oil Recovery and Reuse, 1978). However, the distillate has to be further processed in order to make an end-product equivalent to the virgin basestock in quality performance. Heat soaking is also known in the industry as a method for breaking up additive molecules and precipitating a polymer (See U.S. Pat. No. 4,033,859). Predistilling a used oil preferably by steam stripping said oil, within the temperature range of between 480.degree. F. and 650.degree. F. (about 249.degree.-345.degree. C.) for at least 4 hours to remove NO.sub.x, light oil components and residual water from the stock prior to distilling the oil in a thin film evaporator is described in U.S. Pat. No. 4,101,414.